This invention relates to the stabilization of microbicides. In particular, this invention relates to the improved stabilization of 3-isothiazolone dilute solution compositions.
Microbicides are used commercially to prevent the growth of microbes in a variety of loci, such as cooling towers, metal working fluid systems, paint and cosmetics. One of the more important classes of microbicides is 3-isothiazolones. Many 3-isothiazolones have achieved commercial success because they are very effective in preventing microbial growth under a wide variety of conditions and in a variety of loci. Among the most important 3-isothiazolones are 5-chloro-2-methyl-3-isothiazolone, 2-methyl-3-isothiazolone, and mixtures thereof.
While 3-isothiazolones are very effective microbicides, they suffer from being unstable under certain conditions. Without the presence of a stabilizer, many 3-isothiazolones chemically degrade and lose microbicidal efficacy. Much research has been devoted to stabilizing 3-isothiazolones.
In general, compounds that stabilize 3-isothiazolone concentrates do not stabilize 3-isothiazolone dilute solutions. Compounds, such as magnesium nitrate, that do stabilize both 3-isothiazolone concentrates and dilute solutions do so in greatly differing amounts. More magnesium nitrate is required to stabilize a 3-isothiazolone dilute solution than a concentrate; 23 percent by weight for dilute solutions as compared to 12 to 16 percent by weight for concentrates. As dilute solutions are typically prepared by diluting 3-isothiazolone concentrate compositions, this need for additional stabilizer results in increased cost and handling.
Typical 3-isothiazolone products of a 3:1 mixture of 5-chloro-2-methyl-3-isothiazolone and 2-methyl-3-isothiazolone contain between 1 and 25 percent by weight of the 3-isothiazolone mixture and a similar amount of a stabilizer. Concentrate compositions of a 3:1 mixture of 5-chloro-2-methyl-3-isothiazolone and 2-methyl-3-isothiazolone generally contain about 5 to 35 percent by weight of the 3-isothiazolone compounds and require about 10 to 25 percent by weight of a stabilizer, such as magnesium nitrate. Dilute solutions of a 3:1 mixture of 5-chloro-2-methyl-3-isothiazolone and 2-methyl-3-isothiazolone contain about 0.5 to 5 percent by weight of the 3-isothiazolone compounds.
Dilute solutions of 3-isothiazolones are usually stabilized either with high levels of magnesium nitrate (23 wt %), with a combination of magnesium nitrate (1.5-5 wt %) and very low levels of copper nitrate (0.037-0.14 wt % as copper ion), with a combination of magnesium nitrate (4-5 wt %) and copper sulfate (4 wt %), or with a low levels of hydrogen peroxide.
These known stabilized 3-isothiazolone dilute solutions suffer from having a high metal salt content or having limited stability. When a 3-isothiazolone stabilized with a metal salt is added to a latex formulation, the high metal salt content can coagulate the latex. Although the above described stabilizers for 3-isothiazolone dilute solutions allow the 3-isothiazolones to retain their microbicidal efficacy for considerable periods of time, they do not prevent other problems from developing, such as the formation of precipitate upon storage.
The presence of this precipitate does not impact the efficacy of the 3-isothiazolones; however, the presence of the precipitate gives an undesirable appearance to users of the product. It is clearly preferable from a commercial standpoint to have a product which does not form a precipitate.
U.S. Pat. No. 5,461,150 (Gironda, et al.), herein incorporated by reference, discloses the stabilization of 3-isothiazolone concentrates with a low level of cupric ion.
While these compositions are stable, they suffer from the formation of a brown precipitate upon storage. Such brown precipitate is particularly undesirable when such compositions are used to preserve cosmetics and toiletries. Gironda et al. do not address the problem of precipitate formation upon storage of the 3-isothiazolone compositions.
Thus, there is a continuing need for stable 3-isothiazolone dilute solution compositions that remain stable and do not form precipitate upon storage.